This seems like another amazingly vague question. Was the traffic constant? Did you change anything, like switching to image ads or flash ads or text ads? What else was happening at the same time?
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deleteJul 11 '10 at 2:23

Traffic constant, and i didn't change anything at all
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jfoucherJul 11 '10 at 7:16

It may be due to what is called Google Panda. A few months back Google changed their algorithm due to complaints that their search results weren't "up-to-par." A ton of websites were negatively affected by this change, being pushed down in the SERPs. I believe in June or July sometime, they made the change effective worldwide. Perhaps your AdSense earnings drop is due to lower traffic, which is, in turn due to the changes in Google's algorithm.

Has your traffic changed or remained the same?

Check out your main keywords in Google Adwords Keyword tool, have their CPCs dropped? This can definitely happen -- it's happened to me! That could be a huge factor in why you're seeing lower returns in Google AdSense.

Firstly there might not be a specific reason, it could just be due to random fluctuations/variance. Perhaps you need a longer period of time, and more data (larger sample space) to do your comparisons.

But assuming there is a specific reason for the traffic drop, try to analyze the data you have via your web stats (Google analytics or something similar). If you don't have any you should consider getting one setup.

Specifically look for if you used to have any traffic sources, or keywords that were brining in more traffic than now. You mentioned your traffic has remained constant, but perhaps some of your traffic sources that performed better are getting less hits compared to the others.

Also you might want to check how targeted your ads are. Perhaps some content that you posted recently is causing a less targeted ad. It's common for non targeted ads for your audience to have a significant difference (even 100x difference) in performance.

Google and other search engines can also tweak their search engines causing significant difference in traffic at any time, but you mentioned your traffic has remained constant.